Recent discoveries of high pressure oil and gas reserves in the Gulf of Mexico and the North Sea have presented a challenge to subsea production control as the initially encountered well pressure is very high but later expected to significantly drop over time.
Currently, pressure and flow rate control is achieved using a single subsea production choke mounted on a subsea production tree. However, as the excess pressure in HP wells may be as high as 5000 to 6000 psi across the production choke, rapid deterioration or even failure of the choke is likely due to the severe operating conditions at the choke trim. An exemplary subsea choke valve is described in U.S. Pat. No. 4,589,493, which is incorporated by reference herein, and improvements to alleviate at least some of the difficulties associated with product flow characteristics near the off position are shown in U.S. Pat. No. 6,701,958. As the production stream contains in addition to gas and crude oil also particulate matter, operation at relatively high pressure often severely reduces the lifetime of choke valves due to mechanical wear.
Wear resistance can be improved by using disk stacks in which multiple disks define a 3-dimensional tortuous path through which the high-pressure fluid is routed. Examples for such choke valves are disclosed in U.S. Pat. No. 4,938,450 and WO 2007/074342. While such choke valves significantly improve wear resistance and cavitation, several problems still remain. Among other things, large pressure differentials are often difficult to control with such valves. Alternatively, the high-pressure fluid may be fed through a series of concentric sleeves that define flow paths by inclusion of sleeve openings, wherein the sleeves can be rotated relative to each other to thereby narrow or widen the flow path. Representative examples of such choke valves are described in U.S. Pat. No. 5,018,703. In other known configurations, and in further attempts to reduce wear and adverse effect of pressure, flow may be directed in a radial manner and redirected by baffles as described in U.S. Pat. No. 6,105,614. However, as in the choke valves before, large pressure differentials are difficult to control with such known devices.
Pressure differences in high pressure oil and gas fields at early production are often estimated to be around 6000 psi or even higher, but then expected to substantially decrease over time. Such anticipated pressure gradient is difficult to manage in a safe and economic manner using currently known technology. Among other reasons, current production chokes may have a flow coefficient Cv of 1 GPM*psi−0.5 when the choke is at or near closed position, which corresponds to a rate of 3000 BBLs per day liquid rate. However, the well will require a very high Cv in later production to compensate for the much lower well pressure. Therefore, the ideal choke valve should have a low Cv in beginning of well production and a high Cv in late well production to allow for sufficient production control without costly intervention or choke replacement. Unfortunately, while wide range Cv valves were suggested, commercially and technically feasible wide range Cv valves have not been developed.
To overcome such problems with a wide range of Cv, it was proposed to employ a topside choke in combination with a subsea choke. While the combination of a subsea production choke in combination with a topside choke advantageously provides a widened control of Cv, numerous new difficulties arise. For example, such configurations require high-pressure flowlines boarding the production vessel, which presents a significant risk during equipment failure. Alternatively, it was also proposed that a second choke could be mounted on the production deck or at the subsea riser base. While such configurations would reduce severity of service conditions at the chokes, subsea flowlines must then accommodate high pressure, adding risk and capital cost to the project. Worse yet, in case of equipment failure, substantial hazards to platform and personnel nearby or on the production deck may exist. Still further, elevated pressure in the flowlines will pose substantial challenges for flow assurance due to higher risk of hydrates formation and plugging.
Multiple choke configurations are known for downhole applications in which each of the chokes is separately controlled and in which the chokes are arranged in parallel as described in U.S. Pat. App. No. 2007/0163774. Control systems for such downhole multi-choke devices is typically in electrohydraulic manner as described in WO 99/47788. However, the chokes in such configurations are predominantly used to isolate areas within a well, for example, to reduce or prevent water intake in a production line. Consequently, such chokes will operate in an on/off manner and typically not allow for flow control.
Therefore, while numerous configurations and methods of production well control are known in the art, all or almost all of them suffer from one or more disadvantages. Thus, there is still a need to provide improved configurations and methods of production well control.